There are demands in the optical transmission system for an increase in communication capacity, a reduction in communication cost, an increase in transmission speed, and extension of relay intervals, due to a rapid increase in the number of subscribers for the Internet portable telephones and the like, and an increase in needs for speech communication, image communication and the like.
There is a one-fiber bidirectional optical transmission system for realizing these. This system can transmit output light signals from optical transmitter-receivers, respectively connected to the opposite ends of one optical fiber transmission line, bidirectionally into the optical fiber transmission line, thereby reducing the number of the optical fibers and improving the use efficiency of the optical fiber. As a result, an increase in the communication capacity, a reduction in the communication cost, and an increase in the transmission speed can be realized.
There is also an optical transmission system using a Raman amplifier. This system uses a Raman amplification effect, which is obtained when a pump light is incident on the optical fiber serving as an amplification medium. Extension of relay intervals and a reduction in the communication cost can be realized by using the amplification effect and increasing the length of the optical fiber transmission line.
Raman amplification is a phenomenon in which the optical fiber itself where a signal light is transmitted is used as an amplification medium, a pump light incident on the optical fiber causes vibrations in a crystal lattice of a material forming the optical fiber, and due to the interaction between the pump light and optical phonons generated by the vibrations in the crystal lattice, scattered light referred to as Stokes light is induced to a short frequency shifted by a peculiar quantity from the pump frequency, and amplified. The amplification gain generated by the Raman amplification depends on the material of the optical fiber, and generally has a Raman gain band as shown in FIG. 9. FIG. 9 is a graph of a gain band of a typical Raman amplifier, wherein X axis denotes a wavelength difference (nanometer), and Y axis denotes a Raman gain coefficient. A wavelength having the maximum gain is on the long wavelength side apart from a pump wavelength by 100 nanometers to 110 nanometers, and has a gain band in a wavelength range of about 60 nanometers, with the bottom widely extending from the central wavelength having the maximum gain towards the short wavelength side.
As for the incident direction of the pump light in Raman amplification, there are two types, that is, a method in which the pump light enters in the same direction as the traveling direction of a signal light (forward pumping), and a method in which the pump light enters in a direction opposite to the traveling direction of the signal light (backward pumping). In general, however, it is known that the backward pumping with less crosstalk generated in a signal light by the pump light is advantageous. Therefore, an optical transmission system obtained by combining the one-fiber bidirectional optical transmission system with the backward pumping Raman amplifier is expected as an optical transmission system utilizing its merits.
In the one-fiber bidirectional optical transmission system, however, multiplexed signal lights are transmitted bidirectionally, and a plurality of pump lights is used, and hence there is a problem in that the system is likely to be affected by four wave mixing. The four wave mixing is a phenomenon in which optical signals having different wavelengths, which propagate in the optical fiber transmission line, affect each other to generate light having a new wavelength.
FIG. 10(a) is one example of reception spectra after being transmitted for 200 kilometers, when wavelength-multiplexed signal lights and a Raman pump light are input to the optical fiber transmission line. FIG. 10(b) is an enlarged diagram thereof in the vicinity of the wavelength-multiplexed signal lights in FIG. 10(a). Specific parameters are such that the wavelengths of backward pumped Raman pump light are 1430 nanometers and 1460 nanometers, the wavelengths of the wavelength-multiplexed signal lights are 32 wavebands of from 1576.2 nanometers to 1602.3 nanometers (with an interval of 100 gigahertz), and the optical fiber transmission line is a non-zero dispersion shifted fiber with a zero dispersion wavelength at 1505.2 nanometer waveband.
As shown in FIG. 10(b), the signal level is raised at a wavelength in the vicinity of 1589 nanometers, thereby causing deterioration in the frequency-multiplexed signal. This is because four wave mixing occurs due to the Raman pump light and the frequency-multiplexed signal lights propagating in the same direction, thereby causing phase matching with a zero dispersion wavelength in the optical fiber transmission line. In an example shown in this figure, a frequency fp of the Raman pump light is 209.65 terahertz (1430 nanometers), one signal frequency fs of a frequency-multiplexed signal light is 188.70 terahertz (1588.7 nanometers), and a zero dispersion frequency f0 in the optical fiber transmission line is 199.17 terahertz (1505.2 nanometers), and at this time, the phase matching condition of a following equation|fp−f0|=|fs−f0|  (1)is realized, and deterioration in the frequency-multiplexed signal lights occurs due to four wave mixing.
This is a phenomenon in which deterioration in optical signals occurs, when a light generated by the four wave mixing and an input signal light satisfy a certain phase matching condition, and it becomes an important subject how to overcome the deterioration in the optical signal.
It is therefore an object of the present invention to provide a one-fiber bidirectional optical transmission system that can obtain desired reception characteristics, without being affected by four wave mixing bidirectionally.